In order to use the available spectrum resources flexibly and communicate without interfering the communication of primary users, the cognitive radio networks should have an efficient and reliable control information exchange mechanism so as to exchange all kinds of control information including spectrum sensing result, network topology, clock synchronization, and communication resources reservation among cognitive nodes. The control information exchange mechanism based on frequency hopping can flexibly and quickly find out idle communication frequency bands, which are not occupied by the primary users, through continuously changing the communication frequency band (or channel). Thus, the interference of the communication of primary users can be addressed properly.
During the control information exchange based on frequency hopping, only when at least one transceiver of the cognitive sending node and at least one transceiver of the cognitive receiving node achieves the frequency rendezvous. i.e., the transceivers of the two nodes hop to the same channel in the same timeslot, the two nodes can realize the exchange of the control information. In order to ensure that the exchange of the control information in the cognitive radio network operates smoothly, each cognitive node needs to provide a frequency hopping sequence for each configured transceiver thereof. Moreover, the frequency hopping sequences provided for all transceivers of the node form a frequency hopping sequence set. When all cognitive nodes in the entire network generate the frequency hopping sequences based on the same rule, all frequency hopping sequence sets generated based on the rule form a frequency-hopping system. If each cognitive node selects the frequency hopping sequence set from the frequency-hopping system independently and randomly, any two cognitive nodes can always exchange the control information based on the frequency hopping rendezvous without the prior knowledge of their frequency hopping sequences. As different pairs of cognitive nodes can rendezvous at different channels in the same timeslot, the problem of the saturation of a single fixed control channel caused by the control information exchange of the cognitive radio network can be efficiently avoided. Therefore, how to design a frequency-hopping system for the cognitive radio network, which can be used by all cognitive nodes of the entire network, plays a key role in the performance of the control information exchange of the cognitive radio network based on the frequency hopping rendezvous.
Usually, parameters to evaluate the performance of a frequency-hopping system include:
Degree of rendezvous (DoR): the total number of channels in which any two frequency hopping sequence sets in the frequency-hopping system can rendezvous. Normally, larger the DoR, more reliable is the control information exchange based on frequency hopping rendezvous, and stronger is the anti-jamming ability against the primary user.
Average time-to-rendezvous (ATTR): the average time interval between two consecutive rendezvouses of any two frequency hopping sequence sets in the frequency-hopping system. Normally smaller the ATTR, shorter is the average delay of control information exchange based on frequency hopping rendezvous, and better is the performance of the control information exchange.
Maximum time-to-rendezvous (MTTR): the maximum time interval between two consecutive rendezvouses of any two frequency hopping sequence sets in the frequency-hopping system. Normally, smaller the MTTR, shorter is the maximum delay of control information exchange based on the frequency hopping rendezvous, and better is the performance of the control information exchange.
Channel loading (CL): the ratio between the maximum number of frequency hopping sequence sets which can rendezvous in the same channel in the same timeslot, and the total number of the frequency hopping sequence sets in the frequency-hopping system. Obviously, the CL is within the range of [0, 1]. Normally, smaller the CL, lesser is the transmission collisions or congestions generated by control information exchange on the frequency hopping rendezvous in the cognitive radio network, and better is the performance of the control information exchange.
Number of sequences per set (NSS): the number of frequency hopping sequences contained in each frequency hopping sequence set of the frequency-hopping system. Since the cognitive node needs to provide one individual transceiver for each frequency hopping sequence in the frequency hopping sequence set, NSS actually represents the minimum number of transceivers that the cognitive node should have. Normally, smaller the NSS, fewer are the transceivers needed to achieve the control information exchange based on the frequency hopping rendezvous, and lower is the hardware complexity of the cognitive node.
Obviously, the theoretical lower bounds of the MTTR and ATTR of a frequency-hopping system both are 1. That is, any two frequency hopping sequence sets in the frequency-hopping system can always achieve the rendezvous and exchange the control information in any timeslot. On the other hand, under the limiting condition that the clock of the cognitive radio network node may be asynchronous, even if all cognitive nodes use the same frequency hopping sequence set, two cognitive nodes may not achieve the frequency hopping rendezvous on all accessible channels due to the difference of the start time of the frequency hopping.